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ECE669 L3: Design Issues February 5, 2004 Dataflow Architectures °Represent computation as a graph of essential dependences Logical processor at each node, activated by availability of operands Message (tokens) carrying tag of next instruction sent to next processor Tag compared with others in matching store; match fires execution

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ECE669 L3: Design Issues February 5, 2004 Evolution and Convergence °Key characteristics Ability to name operations, synchronization °Problems Operations have locality across them, useful to group together Handling complex data structures like arrays Complexity of matching store and memory units Expose too much parallelism °Converged to use conventional processors and memory Support for large, dynamic set of threads to map to processors Typically shared address space as well Separation of progr. model from hardware °Lasting contributions: Integration of communication with thread (handler) generation Tightly integrated communication and fine-grained synchronization

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ECE669 L3: Design Issues February 5, 2004 SAS Programming Model °Naming: Any process can name any variable in shared space °Operations: loads and stores, plus those needed for ordering °Simplest Ordering Model: Within a process/thread: sequential program order Across threads: some interleaving (as in time-sharing) Additional ordering through explicit synchronization

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ECE669 L3: Design Issues February 5, 2004 Synchronization °Mutual exclusion (locks) Ensure certain operations on certain data can be performed by only one process at a time Room that only one person can enter at a time No ordering guarantees °Event synchronization Ordering of events to preserve dependences -e.g. producer —> consumer of data 3 main types: -point-to-point -global -group

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ECE669 L3: Design Issues February 5, 2004 Message Passing Programming Model °Naming: Processes can name private data directly. No shared address space °Operations: Explicit communication through send and receive Send transfers data from private address space to another process Receive copies data from process to private address space Must be able to name processes °Ordering: Program order within a process Send and receive can provide pt to pt synch between processes °Can construct global address space: Process number + address within process address space But no direct operations on these names

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ECE669 L3: Design Issues February 5, 2004 Design Issues Apply at All Layers °Programming model’s position provides constraints/goals for system °In fact, each interface between layers supports or takes a position on: Naming model Set of operations on names Ordering model Replication Communication performance °Any set of positions can be mapped to any other by software °Let’s see issues across layers How lower layers can support contracts of programming models Performance issues

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ECE669 L3: Design Issues February 5, 2004 Ordering °Message passing: no assumptions on orders across processes except those imposed by send/receive pairs °SAS: How processes see the order of other processes’ references defines semantics of SAS Ordering very important and subtle Uniprocessors play tricks with ordering to gain parallelism or locality These are more important in multiprocessors Need to understand which old tricks are valid, and learn new ones How programs behave, what they rely on, and hardware implications

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ECE669 L3: Design Issues February 5, 2004 Replication °Reduces data transfer/communication depends on naming model °Uniprocessor: caches do it automatically Reduce communication with memory °Message Passing naming model at an interface receive replicates, giving a new name Replication is explicit in software above that interface °SAS naming model at an interface A load brings in data, and can replicate transparently in cache No explicit renaming, many copies for same name: coherence problem In uniprocessors, “coherence” of copies is natural in memory hierarchy

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ECE669 L3: Design Issues February 5, 2004 Communication Performance °Performance characteristics determine usage of operations at a layer Programmer, compilers etc make choices based on this °Fundamentally, three characteristics: Latency: time taken for an operation Bandwidth: rate of performing operations Cost: impact on execution time of program °If processor does one thing at a time: bandwidth  1/latency But actually more complex in modern systems °Characteristics apply to overall operations, as well as individual components of a system